Method of fabricating plain bearings

ABSTRACT

A lead-free multilayer material for plain bearings and a process for the production of corresponding bearing shells for improved emergency running and mechanical load-carrying capacity characteristics. The multilayer bearing material comprises a lead-free copper alloy having a copper content ranging from 50 to 95 wt. %, and an overlay formed of a lead-free tin and copper having a tin content ranging from 70 to 97 wt. % and a copper content ranging from 3 to 20 wt. %.

This application is a division of application Ser. No. 09/108,785, filed Jul. 2, 1998.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a composite multilayer material for plain bearings, comprising a ternary layer, a bearing metal layer, a diffusion barrier layer and an overlay applied by electroplating. The invention also relates to a process for the production of bearing shells.

2. Description of Related Art

Composite multilayer materials with a structure comprising steel backing/lead-bronze/lead-tin-copper overlay have proven very reliable and to have a high mechanical load carrying capacity. Such composite multilayer materials are described in Glyco Ingenieurberichte 1/91, for example.

A typical representative of these groups of materials has the following structure:

steel

CuPb22Sn bearing metal

nickel barrier

PbSn10Cu2 overlay.

The electrodeposited overlay in particular is a multifunctional material, which, inter alia, takes on the following tasks:

embeddability with respect to foreign particles

running-in or conformability of sliding counterparts

corrosion protection for the lead-bronze

emergency running properties in the case of an oil shortage.

The bearing metal likewise contains certain emergency running reserves in case the overlay is completely worn away.

These bearing designs, which have proven effective for decades, today still contain quantities of lead in the bearing metal and the overlay. This is unsatisfactory from the point of view of the environmental impact of heavy metals.

The only lead-free plain bearings currently produced are aluminium-based. Two-component bearings comprising an aluminium alloy on steel are preferably used. Alloy elements in the aluminium are preferably tin, silicon, nickel and copper. Disadvantages of these groups of materials are their low fatigue strength, low notch sensitivity, low heat resistance, insufficient embeddability and relatively poor heat conductivity.

Electrodeposition has hitherto mainly been carried out using baths containing fluoroborate. Copper deposition could be carried out only up to a rate of 2 vol. % in these baths, while in cyanide baths deposition rates for the copper of up to 20 vol. % could be achieved. However, it has become clear that the coating is extremely brittle and in this respect is not very durable.

DE-OS 27 22 144 discloses the use, as a soft metal bearing alloy for multilayer plain bearings, of an alloy comprising more than 6 to 10 wt. % copper and 10 to 20 wt. % tin, the rest being lead. This alloy may, inter alia, be applied by electrodeposition, wherein a nickel intermediate layer is provided as a diffusion barrier. However, this known alloy, which is produced using conventional electrolyte baths, exhibits coarse tin distribution.

The coating of electronic components is described in “galvanisches Abscheiden von Zinn-Blei aus Fluoroborat und fluorboratfreien Elektrolyten” (“Electrodeposition of tin-lead from fluoroborate and fluoroborate-free electrolytes”) by H. van der Heijden in “Metalloberfläche” (“Metal surface”) 39 (1985) 9, pages 317-320. It has been shown that tin and lead may be easily dissolved in various organic sulphonic acids with the general formula RSO₃H, wherein R represents an alkyl group. The sulphonic acids are described as completely stable during electrolysis. The use of such fluoroborate-free baths for electroplating of sliding elements is mentioned, as are suitable additives necessary for the production of sliding elements.

Aqueous, acidic solutions are known from DE 39 02 042 A1 for the electrolytic deposition of tin and/or lead-tin alloys. These binary electroplating baths are used as corrosion protection, e.g. for printed circuit boards and batteries, wherein a relatively soft layer is deposited which is not wear-resistant and thus cannot be used for sliding elements. In addition to metal salts, inhibitors and free alkanesulphonic acids, these known electroplating baths comprise brighteners, which make it impossible to use these baths for example as ternary baths. It has been shown that, when copper salts are added for example, only a maximum of 1% copper may be deposited, because the bath composition is essentially suited to tin. JP 02/93096A (Pat. Abstract of Japan, C-732, Vol. 14/No. 294) describes a process for producing sliding elements, in which the overlay of Pb—Sn—Cu is applied to the prefabricated semi-finished product by electroplating. To this end, a fluoroborate-free electrolyte is used which does not contain brighteners but does contain free alkylsulphonic acid and non-ionic wetting agents. With these known electroplating baths, however, again only coarse tin deposition is achieved, wherein the layer structure is generally irregular. The high demands made of plain bearings are not met wholly satisfactorily.

DE 195 45 427 likewise describes fluoroborate-free electroplating baths for the production of overlays of lead-tin-copper, said baths containing a grain-refining agent comprising carboxylic acid and a fatty acid glycol ester. In this way, finely crystalline deposition of the tin is achieved with completely homogeneous distribution thereof, copper deposition of from 2 to 16 wt. % being obtained.

The prior German Application DE 196 22 166 describes ternary layers with hard particles, wherein fluoroborate-free electroplating baths with added alkylsylphonic acid are used for the overlay, to obtain a completely homogeneous distribution of the hard particles in the matrix material. In addition to lead-containing overlays, these alkylsulphonic acid baths may also be used for the application of lead-free layers of SnCuNi, Sn, SnCu and CuSn. However, it has been shown that, although copper contents of up to 16% may be obtained with these lead-free layers, these layers do not exhibit satisfactory properties as far as mechanical load carrying capacity and fatigue strength are concerned.

SUMMARY OF THE INVENTION

The object of the invention is therefore that of providing a lead-free composite multilayer material which exhibits at least the positive characteristics with respect to reliability, emergency running properties and mechanical load carrying capacity which are exhibited by conventional lead ternary bearing materials. It is also the object of the invention to provide a process for producing bearing shells.

DETAILED DESCRIPTION OF

THE PREFERRED EMBODIMENTS This object is achieved, with regard to the composite multilayer material, in that the bearing metal layer consists of a lead-free copper alloy with a copper content of from 50 to 95 wt. % and in that the overlay consists of a lead-free alloy comprising tin and copper, the copper content amounting to from 3 to 20 wt. % and the tin content to from 70 to 97 wt. %.

Lead-free copper alloys are already known from bush production (c.f. “Lagerwerkstoffe auf Kupferbasis” (“Copper-based bearing materials”) Deutsches Kupferinstitut e.V. 1961) and have proven effective. However, bushes are exposed to lower mechanical loads and lower rotational speeds than bearing shells. For this reason, consideration has not hitherto been given to the use of these materials for bearing shells. However, it has emerged that such copper materials are suitable for bearing shells if an overlay is additionally applied.

Since it was also necessary to omit lead from the overlay, there was not complete freedom of choice with regard to the overlay material to be used in conjunction with the lead-free bearing material. It has emerged that tin-copper alloys exhibit the best results in relation to corrosion resistance, load carrying capacity and embeddability when the copper content is between 5 and 20%.

The bearing metal layer preferably consists of a copper-aluminium, copper-tin, copper-zinc, copper-zinc-silicon, copper-zinc-alumium nic or copper-aluminium-iron alloy. Examples are CuA18, CuSn8, CuZn33, CuZn31Si, CuZn40A12 or CuAl10Fe.

The overlay may contain bismuth and/or silver and/or nickel each in a proportion of no more than 20 wt. %.

The overlay may additionally comprise hard particles, wherein Al₂O₃, Si₃N₄, diamond, TiO₂ or SiC are possible materials. These hard particles may be included in the overlay on their own or in combination.

The diffusion barrier layer may consist of a nickel layer from 1 to 3 μm thick and a nickel-tin layer from 2 to 10 μm thick deposited thereon. In another variant the diffusion barrier layer consists of cobalt, which may likewise have a thickness of from 1-3 μm.

The process for producing bearing shells provides that firstly the bearing metal of lead-free copper material is cast onto steel strip, wherein it may be necessary to provide wetting agent layer s of nickel, thin or copper on the steel strip depending on the wettability of the bearing metal melt with respect to the steel strip. Such layers are produced in electroless manner, by electroplating or by the molten state method and are preferably from 1 to 20 μm thick.

Adjustment of the fine-grained structure of the bearing metal is effected by a combination of metal working and heat treatment processes.

After this thermomechanical treatment of the composite material, bearing shells are pressed therefrom. The bearing shells are provided with a diffusion barrier layer of nickel, nickel/nickel-tin or cobalt and a tin-based lead-free overlay by electroless chemical or electrochemical processes.

A ternary, fluoroborate-free electroplating bath is used from which brighteners are omitted but to which there are added non-ionic wetting agents and free alkylsulphonic acid, a fatty acid polyglycol ester and a grain-refining agent comprising a carboxylic acid also being used.

Hard material particles are dispersed in the bath especially to increase wear-resistance. The addition of suitable wetting agents to the bath makes it possible to codeposit the particles with the overlay.

The percentages by volume of the dispersate amount to up to 20%, the particle size amounting on average to ≦1 μm.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise that specifically described. 

What is claimed is:
 1. A method of fabricating plain bearings comprising: preparing a metallic backing layer; forming a lead-free bearing material by casting a lead-free copper alloy onto the metallic backing layer having a copper content ranging from about 50 to 95 wt. %; forming a diffusion barrier layer on the bearing metal layer; and electroplating a lead-free overlay onto the diffusion barrier layer comprising a Sn-Cu alloy having a copper content ranging from about 3 to 20 wt. % and a tin content ranging from about 70 to 97 wt. %.
 2. The method of claim 1 further characterized by electroplating of the overlay in a fluoroborate-free electroplating bath that is free of brighteners and which includes at least one non-ionic wetting agent, free alkylsulphonic acid, a fatty acid polyglycol ester, and a grain-refining agent comprising carboxylic acid. 